1. Field of the Invention
The present invention generally relates to a transmission for a four-wheel drive vehicle and a four-wheel drive vehicle having the transmission, and particularly relates to a transmission suitable for a four-wheel drive vehicle having a swing arm for the rear wheels, such as an all-terrain vehicle (A.T.V.), a utility vehicle for transport of articles, and the like, which is suitable for traveling on rough road.
2. Description of the Related Art
Conventionally, four-wheel drive vehicles having rear wheels supported by a swing arm system for traveling on rough road have been widely prevalent. In such a four-wheel drive vehicle having the swing arm system, the engine and the transmission have conventionally been mounted on a body frame of the vehicle. And in such a conventional four-wheel drive vehicle, the transmission on the body frame and a front reduction gear mechanism built in a front reduction gear case provided between two front wheels are connected to each other through a front propeller shaft for driving the front wheels, and the transmission and a rear reduction gear mechanism built in a rear reduction gear case mounted on the swing arm are connected to each other through a rear propeller shaft for driving the rear wheels.
For example, a four-wheel vehicle with a swing arm system in which the rear wheels are driven has been conventionally known, as disclosed in Japanese Patent No. 2694202. Also, as the transmission for the four-wheel drive vehicle where the transmission is mounted on the body frame of the vehicle, a power takeoff shaft of the transmission, extending in the direction of the front and rear of the vehicle, being mounted on one side of the transmission or under the transmission, in which the front part of the power takeoff shaft is connected to the front propeller shaft for driving the front wheels, and in which the rear part of the power takeoff shaft is connected to the rear propeller shaft for driving the rear wheels, has been disclosed in Japanese Laid-Open Patent Publication No. 2002-227939.
According to the four-wheel drive vehicle in which the rear axle shaft is supported by the swing arm system and in which the engine and the transmission are mounted on the body frame, the location of the engine and the transmission is necessarily somewhere in a front half of the vehicle, for example, under the seat. With this location, the vibration from the engine is easily transmitted to the driver, the space around the driver becomes smaller, and therefore the comfort of riding the vehicle is lowered.
Also, the rear propeller shaft which connects the transmission on the body frame to the rear reduction gear mechanism on the swing arm is mounted so as to be able to swing up and down along with the swing of the swing arm. Therefore, compared with the front propeller shaft, the loss of power transmitting by the rear propeller shaft for driving the rear wheels becomes greater than that of the former. Namely, if the vehicle is a utility vehicle for the transport of articles that is loaded with heavy goods at the rear, for example, the load which the rear wheels receive becomes greater. Therefore, the economical efficiency becomes lower in view of its incurred fuel consumption.